The present invention relates to an external sensor which may be detachably electrically connected to an electronic flash of the type including a flash duration control circuit for controlling the flash duration of a flash tube which flashes by discharging the electrical energy stored on a main capacitor, the external sensor including a light receiving unit which generates the flash interrupt signal in response to which said flash duration control circuit is activated when the amount of received light reaches a predetermined level.
An external sensor of the type described is disclosed in detail in for instance U.S. Pat. No. 3,774,072. The external sensor is mounted on a camera and is electrically connected to an electronic flash which is spaced apart from the camera so that a subject may be illuminated by the light emitted from the electronic flash and bounced from walls and ceilings. In the "automatic flash exposure mode", when the amount of light reflected back from the subject and received by the external sensor reaches a predetermined level, the external sensor generates the flash interrupt signal in response to which the electronic flash is deactivated. In the "manual flash exposure mode", the flash duration is controlled manually so that the external sensor must be kept deactivated. Therefore when the "automatic flash exposure mode" is switched over to the "manual flash exposure mode", the external sensor must be mechanically and electrically disconnected from the electronic flash so that the switching over from the automatic mode to the manual mode or vice versa is very cumbersome.
In order to overcome this problem there has been invented and demonstrated an external sensor capable of being switched over from the automatic mode to the manual mode or vice versa, but this sensor is still unsatisfactory in practice because a user tends to fail very often to set the external sensor into a desired mode by operating a mode selection switch which is external to a casing of the external sensor.
In general, the external sensors of the type described above are provided with dials bearing the film speeds, the apertures, the distances to a subject, the guide numbers and so on so that optimum flash exposure data may be selected. However, these exposure data dials are quite ordinarily provided with the whole scales of each exposure data. For instance, a film speed dial may be graduated with ASA 25 to 400, a distance dial, with 4 to 10 meters and an aperture dial, with f2 to f22. Therefore even when specific flash exposure data is determined: the film speed, for example, to be ASA 100, the aperture, f8 and a distance to the subject, 5 meters, all other numbers on these exposure data dials are also visible around the correct numbers. Consequently correct setting and reading of these exposure data dials are rather difficult so that underexposures or overexposures result very often.